The present invention relates to a freeze drying method and apparatus for freeze drying a substance within a freeze drying chamber in which vapor produced by sublimation is condensed within a condensing chamber. More particularly, the present invention relates to such a freeze drying method and apparatus in which the condensing chamber is pressurized prior to the condensation of the moisture in order to help prevent the substance being freeze dried from contaminating the condensing chamber. Even more particularly the present invention relates to such a method and apparatus in which the substance is contained within a solution freeze dried in a bulk freeze drying process involving the freezing of the solution on an array of vertical plates located within the freezing chamber. Still even more particularly, the present invention relates to such a freeze drying method and apparatus that is applied to the decontamination of a solution containing radioactive materials.
Waste disposal problems involving reduction and disposal of radioactive or toxic wastes such as nuclear wastes, wastes containing heavy metals and etc. have long presented an environmental hazard. Such wastes are often processed by dissolving the waste in an acidic solution, for instance nitric acid, and then storing the resultant solution in containers that present a risk of leakage and in any event take up a great deal of storage space. Freeze drying techniques have been applied to such waste disposal problems in order to more properly contain such wastes in a safe and efficient manner. For instance, in U.K. Patent Application GB 2178588, a method and apparatus for treatment for radioactive liquid waste is disclosed in which the radioactive liquid waste is freeze dried to sublimate the solvent and thereby to produce the radioactive solute as a dried deposit.
In any freeze drying process, a substance is frozen within a freeze drying chamber. The substance is then subjected to a reduced pressure while being heated to cause frozen solids to sublimate into vapor. The vapor is condensed within a condensing chamber. In waste disposal applications of freeze drying, it is necessary that the condensing chamber be separated from the freeze drying chamber during the freeze drying process so that condensation chamber does not become contaminated. If such contamination were allowed to occur, radioactively contaminated water would then become a problem which would defeat the whole purpose of the freeze drying process. In order to overcome this problem, in the above referenced U.K. Patent Application, the condensation chamber is segregated from the freeze drying chamber by means of a filter. A filter can, however, limit the size of the freeze dryer because it will reduce the flow of vapor to the condensation chamber.
In the foregoing U.K Patent Application, the freeze drying element located within the freeze drying chamber is a set of pipes. A problem involved with such freeze drying elements is that pipes present a limited surface area and therefore, present another limitation on the size of the freeze drier. Furthermore, any freeze drying element, in addition to presenting a sufficient surface area, must be amenable to removal from the freeze drying chamber for replacement and cleaning purposes.
Although a motivating factor of the present invention is waste treatment, aspects of the present invention have broader applications involving the segregation of the freeze drying process from the environment and the bulk freeze drying of substances within solutions. As will be discussed, the present invention can be generally said to provide a freeze drying method and apparatus in which segregation of the condensation chamber from the freeze drying chamber does not primarily depend on filters. Additionally, freeze drying elements are provided that have sufficient surface area and flexibility for large scale freeze dryer setups.